The invention relates to an anode material for a fuel cell which is to be operated at a high temperature above 700° C., namely to an anode material. The invention also relates to fuel cells with such an anode material. In such fuel cells the anode layer can be applied onto a carrier structure, with an electrolyte layer in particular being formed as a carrier. Or a carrier structure for a thin electrolyte layer is manufactured from the anode material. In the first case, a cathode layer or a foam-like metal layer can be used as a carrier structure instead of the electrolyte layer.
An SOFC fuel cell with a fuel-side carrier structure is known from EP-A-1 343 215 which forms an anode substrate and which serves as a carrier for a thin film electrolyte and also a cathode. In the contact region between the anode, which is a thin part layer of the carrier structure, and the electrolyte, electrochemical reactions take place, at so-called three phase points (nickel/solid electrolyte/gas), in which the nickel atoms are oxidized by oxygen ions (O2−) of the electrolytes and these are then reduced again by a gaseous fuel (H2, CO), with H2O and CO2 being formed and electrons freed during oxidation being conducted further by the anode substrate. EP-A-1 343 215 describes a carrier structure which has a “redox stability” and which with reference to this redox stability is sufficiently well designed with regard to gas permeability and also economics for a use in high temperature fuel cells.
The carrier structure of these known fuel cells is made up of an electrode material and contains macro-pores, which are produced by means of pore formers and form the communicating cavities. The electrode material includes skeleton-like or net-like continuous structure of particles joined by sintering, so-called “reticular systems” (can also be termed percolating phases) which form two interlaced systems: a first reticular system made of ceramic material and a second reticular system which contains metals or one metal—Ni in particular—and which produces an electrically conductive connection through the carrier structure. The electrode material has the characteristics that during the carrying out of redox cycles by means of the change between oxidizing and reducing conditions firstly no substantial changes of characteristic occur in the ceramic reticular system and secondly an oxidation or rather reduction of the metal results in the other reticular system. Moreover, the two reticular systems together form a dense structure which contains micro-pores in the oxidized condition, the proportion of which in relation to the volume of the electrode material is, or can be, smaller than 5% related to the volume of the electrode material.
The two reticular systems arise in a natural way from the constituent particles in the form of a statistical distribution of the particles, if these are prepared in such a way that the two kinds of particles respectively exhibit a narrow size spectrum, when the proportion for each reticular system amounts to 30% per unit volume and when the particles are mixed with each other homogeneously. The system of communicating cavities formed by the macro-pores is likewise a reticular system. This hollow cavity system results in the necessary gas permeability.
The carrier structure described may show the desired redox stability, however in other respects it shows deficiencies. During a redox cycle the structure contracts during the transition from the oxidized state to the reduced state (constriction); the electrolyte layer is correspondingly placed under a compressive pressure. The compression is followed by an expansion during the reversed redox transition. This expansion is greater than the compression by more than 0.01% due to irreversible processes in the carrier structure in many of the anode substrates. Cracks develop in the electrolyte layer, which represents a gas separating membrane, due to the expansion through which the necessary gas tightness is lost.